1. Field of the Invention
The present invention concerns the measurement of pressure inside a steam generator used in a nuclear installation for the production of electricity. It relates more precisely to control of the water level in the secondary circuit of the steam generator.
2. Prior Art and Problem Raised
In steam generators measurement of the water level in the secondary circuit is made by means of several probes or pressure measuring devices so that differential pressure measurements can be made. With reference to FIG. 1, two probes 2S and 2I are shown on the outer wall of a steam generator 1, positioned at different heights. The lower probe 2I is placed just above the generator cyclones, the upper probe 2S being positioned a few meters overhead, vertically above lower probe 2I. This device for measuring differential pressure is completed by a gate 7 on each measuring lane, a condensation pipe 5 downstream from the upper probe 2S surmounted by vent-hole 6, a stabilizing interface 8 and measuring equipment 9.
FIG. 2 shows a section of a steam generator at the level of the lower probe 2I shown in FIG. 1. In fact, several lower probes 21 are used. They lead to inside the steam generator 1. They are covered or surrounded by a tranquilliser 11 of spherical shape. In FIG. 2 manholes 4 are also shown and the outlets of secondary circuit 3.
FIG. 3 shows in more precise manner, and in section view, a probe covered by its tranquilliser. The actual probe is chiefly formed of a sensor pipe 10 fixed onto the inner wall 1I of steam generator 1, whose end part leading into steam generator 1 is surrounded by a spherical-shaped tranquilliser 11. The sensor pipe 10 is closed at its end part but has measuring side holes 12 with which it can sense pressure inside the steam generator. Also, tranquilliser 11 has openings 13 on its sphere allowing the flow of steam to penetrate inside the tranquilliser 11 so that it can be made accessible to sensor pipe 10.
Therefore, several probes, or pressure measuring devices, are placed inside the steam generator so that they can be used to control the pressure inside the generator in order to achieve its optimal functioning.
At certain operating powers of the steam generator, however, in particular at 35% and 100%, thermo-hydraulic phenomena cause clogging of the lower or low pressure probes 2I of an installation for measuring differential pressure, in particular when they have a tranquilliser such as that denoted 10 in FIG. 3. This clogging is mainly due to the depositing of magnetite which forms in the secondary circuit and is carried by the steam. This clogs the openings 13 of tranquilliser 11 and possibly also the side holes 12 of the sensor pipe 10. This clogging causes the condensation pipe 5 in FIG. 1 to fill up, since it prevents the evacuation of return water coming from the condensation pipe, downstream from the measuring equipment. The weight of the column of water thus formed in the measuring equipment is therefore found in the column of water of the low pressure line of the differential pressure sensor. Consequently, the overall measurement of the level inside the steam generator is distorted. This has a strong incidence on the functional safety of the steam generator and on the proper operation of nuclear units.
It is to be noted that these tranquilliser clogging phenomena are particularly apparent on the tranquillisers the furthest from the cyclones in which the steam flow rate vector is oriented differently.
This clogging phenomenon is generally accounted for by the equilibrium existing between the potential energy of the water column increased by reduced pressure in the condensation pipe due to steam condensation, and the kinetic energy of steam. It is specified that overpressures exist below the spherical tranquilliser 13 in FIG. 3, and that reduced pressures exist above it. They are due to the strong current along the walls to which the measuring equipment is fixed. The holes or openings 13 of the tranquilliser 11 may therefore become too small and cause their partial blockage. Since the current inside the sphere forming tranquilliser 13 is relatively low, there may be fouling of the sensor pipe 10.
It is to be noted that this is a problem which dates back as far as 1979.
The purpose of the invention is therefore to remedy these drawbacks by proposing a new type of pressure measuring device intended to measure the differential pressure inside existing steam generators.
For this purpose, the first main object of the invention is a method for measuring pressure inside a steam generator which chiefly comprises:
a sensor tube leading into the steam generator via a closed end part, but provided with side holes;
a tranquilliser surrounding the end of the sensor tube to free the side holes of the sensor tube from harmful dynamic effects produced by surrounding steam currents; and
a condensation pipe on the sensor tube. the method consisting of returning the condensates via the sensor tube.
According to the invention, the method consists of separating the pressure measurement made by the side holes of the sensor tube from the return of condensates via the end part of the sensor tube which is pierced for such purpose with end holes.
In the embodiment of the method of the invention, it is preferable to use a deflector in the tranquilliser in order to set up a chamber free of dynamic pressure opposite these end holes in order to promote emptying of the condensates.
Preferably, the sensor tube is slightly tilted towards the inside of the steam generator to promote the evacuation of the condensates.
The second main object of the invention is a device for measuring pressure inside a steam generator made up of:
a sensor tube leading into the steam generator via a closed end part but which is provided with side holes;
a tranquilliser surrounding the closed end part of the sensor tube and having openings to free the side holes of the sensor tube from the harmful dynamic effects of steam currents in the vicinity of the sensor tube; and
a condensation pipe on the sensor tube, characterized in that, in the tranquilliser, separation means are provided to delimit a measuring channel starting at the side holes of the sensor tube and an emptying channel starting at, at least one, end hole made in the closed end of the sensor tube.
Preferably, these separation means are formed of a separating partition inside the tranquilliser which is of spherical shape, thereby delimiting the emptying channel by a emptying chamber and the measuring channel by a measuring chamber.
In the preferred embodiment of the device of the invention, the measuring channel leads into the steam generator via measuring holes made in the median horizontal plane of the part of the tranquilliser corresponding to the measuring channel.
Preferably, it is of advantage to use a deflector in the tranquilliser, placed opposite at least one end hole made in the end part of the sensor tube so as to form a chamber free of dynamic pressure in the tranquilliser.
It is also preferable to make passage holes in the lower part of the separating partition to promote the evacuation of condensates into the emptying channel.
It is also of advantage, in order to facilitate emptying of the condensates from the condensation pipe, for the sensor tube to be slightly tilted downwards inside the steam generator.
For the purpose of relatively easy maintenance of the device, at least one measuring hole provided in the tranquilliser should be placed opposite a side hole of the sensor tube.
The emptying channel communicates with the inside of the generator via holes which are formed of slatted openings made in that part of the tranquilliser corresponding to the emptying channel.
Provision is made so that the fixation means of the device to the inner wall of the steam generator are formed of at least one magnetic part to partly trap waste circulating in the vicinity of the device.